1. Field of the Invention
This invention relates to a hydrated water-absorption polymer containing resin composition, a porous body (or material) and an insulated wire using the same, a method of making the insulated wire and a coaxial cable.
2. Description of the Related Art
In recent years, according as precision electronic devices and communication devices as used in the medical field are downsized or high-density installed, wires and cables used therefor need to be reduced in diameter.
On the other hand, signal wires are required to provide a further high-speed transmitted signal. Therefore, insulation layer used for the signal wire is desired to be thinned and decreased in dielectric constant as much as possible in order to increase the speed of the transmitted signal.
It is known that insulation materials used therefor are obtained by foaming a low-dielectric constant insulation material such as polyethylene and fluorine resin. It is also known that the foamed insulation materials are formed by winding a pre-foamed film on a conductor or by extrusion. Above all, the extrusion is in wide use.
The foaming technology is generally classified into a physical foaming method and a chemical foaming method.
The physical foaming method is conducted such that a volatile foaming liquid such as liquefied chlorofluorocarbon is injected into a melted resin to make foams by the vaporization pressure, or a foaming gas such as nitrogen gas and carbon dioxide gas is directly injected into a melt resin in an extruder to generate uniformly-distributed cellular fine foams in the resin.
The chemical foaming method is generally conducted such that a resin is molded with a foaming agent dispersed in the resin, it is then heated to cause the decomposition reaction of the foaming agent, and foams are produced by using gas generated by the decomposition.
Related arts to the invention are as follows.
JP-A-S58-62024
JP-A-S57-170725
JP-A-H03-185063
JP-A-H11-5863
JP-A-H11-100457
JP-B-3717942
However, the method of injecting the volatile foaming liquid into the melted rein has the problem that, due to the high-pressure gas, it is difficult to make fine foams and to make a thin mold. Further, due to the low injection speed of the volatile foaming liquid, it is difficult to increase the production speed, where the productivity lowers. The method of directly injecting the foaming gas into the extruder has the problem that it is difficult to produce a small-diameter or thin mold, and it requires a special facility or technology for safety, where the productivity lowers and the production cost increases.
On the other hand, the chemical foaming method has the following problems. After the resin with the foaming agent dispersed therein is molded, it is heated to decompose the foaming agent to produce foams by using the decomposition gas. Therefore, the temperature during the resin molding needs to be lower than the decomposition temperature of the foaming agent. Further, in making the covering by extrusion, small-diameter wires used therefor may be broken by the pressure of the resin, so that it is difficult to increase the production speed.
In addition, the physical foaming method is not environmentally friendly since it uses chlorofluorocarbon, butane, carbon dioxide etc. The foaming agent used for the chemical foaming method is relatively costly.